I need your help to suggest me the best possible solution for upgrading BES 4.1.5 with 400 users in Lotus Domino environment.

In order to understand the complicated scenario, let me first breif you the existing setup in the company:
Firstly, it is a DMZ setup. Currently, BES 4.1.5 is running with 400 users on local MSDE database in Lotus Domino infrastructure. BB router is installed in DMZ. And the strange thing is that the existing BES server in also in DMZ but in different subnet. The installation was done with the local administrator as it is independent workstation and no lotus notes user can be added as it cannot see other servers in the domain. Only port 1352 is open for lotus notes connection.

A similar parallel setup was created in different zone where I installed BES 5.0.3 successfully. But the user migration plan failed with BB Transporter as there is no communication or authentication between the 2 servers.
So now, I have to come out with alternate plans which can be as follows:

1) Bring the existing BES 4.1.5 server inside the LAN so that I can then install the new BES 5.0.3 with it inside the LAN and use BB Transporter to migrate the users from old to new.
Concern: This can be only done if it is confirmed that bringing the old BES server inside the LAN will not cause any interruption to users apart from enabling the port 3101 configuration on the new ip address.

2) Backup the existing database and restore it on central sql server. During fresh BES 5.0.3 installation I will point it to this database and assume that it will ask for upgrade of database and then all the users should work normally.\
Concern: Will I be able to connect to a database of BES version 4.1.5 and upgrade successfully????

3) The last option would be to upgrade the existing BES server itself to 4.1.7 then to 5.0 then 5.0.3.
Concern: This requires lot of interruption and is risky as there are 400 BB users running on it.